The invention relates to a method for improving the total efficiency rate of a combination power plant and to a combination power plant for putting the method into practice.
A large diesel engine, as used herein, refers to a diesel engine that may be used for instance as a main propulsion engine or an auxiliary engine in a ship or in a power plant for the production of heat and/or electricity.
A large diesel engine produces mechanical energy, which may be utilized in a primary circuit for driving a mechanical load, such as an electric generator for transforming the mechanical energy into electricity.
In addition to producing mechanical energy, a large diesel engine discharges a considerable amount of waste heat. For this reason, a secondary circuit may be provided for recovery of waste heat for production of steam either to be utilized in a steam turbine, whereby additional electricity may be produced by means of an electric generator, or to be utilized directly for instance in process industry for different purposes. A combination power plant refers here expressly to these kinds of power plants.
A basic problem in an energy production plant of the kind referred to above is the fact that the temperature of the exhaust gases produced by the engine is relatively low in the secondary circuit. When the recovered heat energy is utilized for steam production, the relatively low temperatures limit the temperature of the steam that is generated, for which reason the efficiency rate of the steam turbine is correspondingly rather low.
Several solutions have been proposed for this problem. One known solution is to arrange for additional burning of the exhaust gases so as to increase the temperature thereof before leading them into a turbocharger. This solution, however, is rather complicated and expensive and requires that additional fuel be supplied for the additional burning.
Another known solution for the problem is described in U.S. Pat. No. 5,133,298, according to which the recovery of the waste energy occurs in two phases. In the first phase before feeding the exhaust gases of the engine into the turbocharger they are led into an exhaust gas boiler, in which part of the heat energy is recovered. If the engine is not operating at its full output rate, some of the exhaust gases may be led past the exhaust gas boiler in a by-pass pipe. Implementation of this solution is costly because the exhaust gas boiler forms an integral part of the exhaust pipe upstream of the turbocharger. In addition the solution takes no account of the reduction in energy of the exhaust gas flow caused by reduction in temperature and pressure when the gas flow passes through the exhaust gas boiler and the by-pass pipe, and the effect of this reduction in energy on the production of energy in the primary circuit, although the matter as such has been grasped in so much as it should be taken account of on the dimensions of the turbocharger.
According to the solution disclosed in the publication FI 94895 all the exhaust gases of the diesel engine are first taken into an exhaust gas boiler for steam production before feeding them into the turbocharger. Although it is possible hereby to increase the total efficiency rate for the production of electricity, the arrangement is awkward and expensive to put into practice, since it requires that substantial arrangements be made upstream of the turbocharger. For the main part the same holds also for the solution disclosed in the publication JP 62-7905.
An aim of the invention is to improve the total efficiency rate for the production of energy in a combination power plant by providing a solution, which is simple and easy to put into practice, which makes uncomplicated arrangements of construction possible and is advantageous as to its costs, and from which the drawbacks in the known technique are essentially eliminated. An aim is especially implementation of the invention in a combination power plant based on one or several diesel engines.